The present invention relates to an image reading apparatus for converting an image of an original into an electric signal.
In a conventional image reading apparatus, for example, in a flat bed type image reading apparatus, an original base made of a transparent plate such as glass is provided on an upper surface of a box type housing. A carriage is provided inside the housing, and this carriage is transported by a drive apparatus in parallel to the original base. Both a light source and a line sensor in which a large number of photoelectric converting elements are arrayed are mounted on this carriage. The irradiation light emitted from the light source is reflected on a surface of an original set on the original base, and then is condensed by a condenser lens onto the line sensor.
A second light source movable in conjunction with the transport of the carriage is provided above the original base in order that a transparency original such as a photographic film is read.
When an image of an original is read, the light emitted from the light source is irradiated onto the original positioned on the original base, and either the reflection light or the transmission light from the original is condensed by the condenser lens onto the line sensor. While the carriage is transported, dark/light information of the original is detected and converted into the electric signal. It should be understood that a direction along which the photoelectric converting elements of the line sensor are arrayed is referred to as a main scanning direction, whereas another direction positioned perpendicular to the main scanning direction, along which both the carriage and the line sensor are traveled, is referred to as a sub-scanning direction.
An output signal derived from the line sensor is amplified by an amplifier and the amplified signal is A/D-converted into a digital signal by an A/D converting unit. In such a case that reading gradation is selected to be 8 bits, this digital signal owns numeral values from 0 to 255.
However, generally speaking, it is practically difficult to irradiate light emitted from a light source onto an original in a uniform light amount, and the light amounts of the light source adjacent to both edges thereof are lowered. Also, since there are fluctuations in the sensitivities of the photoelectric converting elements of the line sensor, even when an original having uniform density is read by this line sensor, the levels of the sensor signal outputted from this line sensor become irregular. There is such a problem that the gradation of this original cannot be correctly reproduced.
To solve this problem, a shading correction is carried out in a shading correcting unit. In accordance with the shading correction, before reading the original, both black reference data and white reference data as to the respective photoelectric converting elements are previously acquired to be stored. A digital light signal is shading-corrected by using these previously stored black/white data based on the following formula: EQU Dn'=A.multidot.(Dn-Dk)/(Dw-Dk),
where symbol "Dn" shows digital signal data before being corrected, symbol "Dk" shows the black reference data, symbol "Dw" represents the white reference data, symbol "A" indicates a constant, and symbol "Dn'" denotes data after being shading-corrected. As to the white reference data, for example, a uniform reflection plane having high reflectivity positioned as a white reference at the edge portion of the original base is read to acquire a sensor signal, and this sensor signal is used as this white reference data. As to the black reference data, when the light source is turned OFF and this uniform plane having high reflectivity is read to acquire a sensor signal, this sensor signal is used as this black reference data. A white reference used to execute the shading correction along the main scanning direction is referred to as a main scanning white reference, or simply as a white reference.
The circuit of the conventional shading correcting unit is indicated in FIG. 10, and this shading correction is carried out in accordance with the following steps a), b), c), and d):
a) The electron charge stored in one element of the line sensor 5 is A/D-converted by the A/D converting unit 12 into the digital signal data Dn. PA1 b) The digital signal data Dn derived from the A/D converter unit 12 is entered into the subtracting circuit 21, and this subtracting circuit 21 calculates a difference "Dn-Dk" between the digital signal data Dn and the black reference data Dk stored in the black reference storage unit 22. PA1 c) The calculation result of the step b) is entered into the dividing circuit 23, and is divided by another difference "Dw-Dk" between the white reference data previously stored in the white reference storage unit 24 and the black reference data to thereby acquire the corrected data Dn'. PA1 d) The steps defined from a) to c) are repeatedly performed as to the respective elements of the line sensor. PA1 a) a step for detecting and setting sub-scanning white reference data when white reference data used in a shading correction is set; PA1 b) a step for detecting sub-scanning white reference data in each of reading lines; PA1 c) a step for setting as a white reference correction coefficient a value indicative of a specific relative relationship between the sub-scanning white reference data when the white reference data is set and the sub-scanning white reference data in the respective reading lines; and PA1 d) a step for reading the original while executing a shading correction with employment of white reference data corrected by the white reference correction coefficient; PA1 wherein, in the case that a plurality of the originals are read, the steps b), c), and d) are repeatedly performed. PA1 a line sensor for converting light from an original into an electric signal; PA1 an A/D converting unit for converting the electric signal derived from the line sensor into a digital signal; PA1 a white reference storage unit for storing white reference data set before the original is read; PA1 a black reference storage unit for storing black reference data set before the original is read; and PA1 a shading correcting unit for shading-correcting an output signal derived from the A/D converting unit with employment of both white reference data corrected by a white reference correction coefficient and the black reference data; wherein the white reference correction coefficient is a value indicative of a specific relative relationship between sub-scanning white reference data when the white reference data is set and sub-scanning reference data in each of reading lines.
As described above, in accordance with the above-explained conventional shading correction, the fluctuations along the main scanning direction can be corrected.
FIG. 11 is a flow chart for explaining one example of an original reading method by the conventional image reading apparatus for executing the above-described shading correction.
First of all, the white reference data is set at a step 101. At a step 102, the original is read while performing the shading correction as explained in the above steps a) to d) with employment of this white reference data. When a decision is made such that the next original is read at a step 103, if it is judged at a step 104 that the light amount of the light source is not varied when the white reference data is set, then the next original can be read by using the white reference data acquired during the previous reading operation while returning to the step 102.
However, in the above-described method, when it is so judged at the step 104 that the light amount is varied, the process operation is returned to the step 101 at which the white reference data is again set. In order to again set the white reference data, it requires a long time period. For example, in the case that an original having a size of A4 is read in reading precision of 600 [dpi], more than 5000 pieces of data must be set as the white reference data. As a consequence, when a plurality of originals are read, there is another problem that the time period required after the reading operation is commenced until this reading operation is completed is prolonged.
Also, the light amount of the light source is varied, depending upon the time elapse after turn-ON operation of the light source is commenced. In general, when time has passed just after the light source is turned ON, the light amount is gradually lowered. As a consequence, the shading corrections are required not only along the main scanning direction, but also along the sub-scanning direction.
Conventionally, the shading correction along the sub-scanning direction is carried out in such a manner that before the read data is digitalized, the analog data are corrected by using the light amount data of the light source, which are sequentially acquired. However, this conventional method owns such a drawback that the difficult control operation is required and also the image quality is readily lowered.
On the other hand, in accordance with the experiments made by the inventors (Applicants) of the present invention, such a fact could be found out. That is, as to the variation in the light amounts of the light source, the variation ratio thereof is substantially constant with respect to the light amount distribution along the main scanning direction.
While using the above-described experimental results, since the white reference data is corrected by a white reference correction coefficient in response to the variation in the light amounts, the re-setting operation of the white reference data can be omitted, and at the same time, the shading correction can be similarly carried out as to the sub-scanning direction.